Enable minimum playtime on with A/B loop points

If set, tracks with A/B loop points set (in library.json) will loop on these
points until they satisfy the minimum playtime, and then continue to the next
track. This has no effect if repeatSingle is true, since that loops A/B
indefinitely.
add shell script to set permissions on audio files

when uploading files over `scp` or `rsync`, the files are not then publicly
accessible. This fixes that.
don't adjust playtime when seeking